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Organic cathode material for high-capacity Li-ion battery with fast charge and discharge

26 November 2012

Nokami
.Extended charge−discharge cycling of PPYT in LiNTf2/G4 at 45 °C, 1 C rate). Credit: ACS, Nokami et al. Click to enlarge.

Researchers from Kyoto University and Panasonic have developed a rechargeable Li-ion battery using a new organic cathode material that exhibits “remarkable” charge–discharge properties with a high specific capacity of 231 mAh/g, excellent rechargeability (83% of the capacity retained after 500 cycles), and charge–discharge ability (90% of the capacity at 30 C as compared to 1 C). A paper on their work appears in the Journal of the American Chemical Society.

Li-ion cathode materials that deliver high power and capacity and that also do not contain heavy metals are highly desired from a viewpoint of sustainability, the team notes in their paper. Organic materials for batteries have received much attention because of their beneficial properties such as light weight, flexibility and availability from easily accessible natural sources.

Although extensive studies have been made, there is still a great demand for organic materials that allow for fast charging and discharging with high cyclability for the storage of electrical energy in practical use. We initiated our project on organic cathode materials for Li-ion batteries based on our experience in organic electrochemistry and organolithium chemistry.

Versatility of chemical structures is a benefit of functional materials based on organic molecules. Although there are various types of redox-active functional groups, it is preferable to choose those consisting of atoms in the second row of the periodic table because of their low atomic weights, availability, and sustainability. Thus, we focus on molecules that consist of hydrogen, carbon, nitrogen, and oxygen, and we found that the polymer (polymethacrylate) bearing pyrene-4,5,9,10-tetraone (PYT) as a redox-active core exhibited remarkable charge−discharge properties as a cathode material in a Li-ion battery.

—Nokami et al.

They designed core structures of Li-ion organic cathode materials based on density functional theory (DFT) calculations, which indicated that six-membered cyclic 1,2-diketones serve as excellent core structures because of the high redox energy change resulting from favorable coordination of the oxygen atoms to Li and the aromaticity of the reduced form.

They chose pyrene-4,5,9,10-tetraone (PYT), which contains two six-membered-ring 1,2-diketone units, a redox core structure because of its favorable capacity.

To prepare the cathodes, they mixed polymer-bound PYT (PPYT) with acetylene black and poly(vinylidene fluoride (PVDF) as a binder. These materials were mixed with (N-methyl-2-pyrrolidone) NMP as a solvent, and the thus-obtained paste was coated on aluminum sheet using a coater. Next, NMP was removed under vacuum at 85 °C for 1 h.

Hermetically sealed two-electrode cells were used for electrochemical experiments. The cathode was separated from the lithium anode by a polyethylene porous film (Celgard) imbued with an equimolar LiNTf2/G4 salt. The three layers were pressed between two current collectors, one in contact with the cathodic material and the other in contact with a lithium disk.

Among their findings:

  • The cell was successively discharged or charged at increasing rates (1 C, 3 C, 5 C, 10 C, 20 C, and 30 C). Even at 30 C, which corresponds to a time of 2 min to fully discharge, the capacity was about 90% of that at the 1 C rate, implying that the battery is suitable for high-power applications.

  • The physical flexibility and affinity to Li ions of methacrylate polymer backbone seem to be responsible for fast charge−discharge ability, although the details are not clear at present.

  • Even after 500 charge−discharge cycles at the rate of 1 C (0.2 C every 20 times), 83% (first cycle, 231 mAh/g; 500th cycle, 193 mAh/g) of the capacity of the material was retained

  • The average Coulombic efficiency between the first and the 500th cycle is 99.96%, which is significantly higher than that of PYT (95.21% between the first and 20th cycle).

High-capacity, fast charge−discharge ability, and excellent cyclability speak well for the high potential of organic materials for Li-ion batteries, and open a new aspect of energy storage. Further work is in progress to explore the detailed mechanism and to develop practical batteries for high-capacity high-power applications.

—Nokami et al.

Resources

  • Toshiki Nokami, Takahiro Matsuo, Yuu Inatomi, Nobuhiko Hojo, Takafumi Tsukagoshi, Hiroshi Yoshizawa, Akihiro Shimizu, Hiroki Kuramoto, Kazutomo Komae, Hiroaki Tsuyama, and Jun-ichi Yoshida (2012) Polymer-Bound Pyrene-4,5,9,10-tetraone for Fast-Charge and -Discharge Lithium-Ion Batteries with High Capacity. Journal of the American Chemical Society doi: 10.1021/ja306663g

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Comments

I must be missing something, but 83% of capacity remaining after 500 cycles does not sound great to me, in fact it sounds pretty lousy.

Most batteries reach that level after the first 20 cycles before stabilizing to a much lower decreasing rate for the next few hundred cycles.

If this battery can be optimized and mass produced at an affordable price, it may represent a worthwhile step towards future enhanced batteries required for extended range BEVs with multiple very quick charge capabilities.

".. 500th cycle, 193 mAh/g - Panasonic have developed a rechargeable Li-ion battery using a new organic cathode material that exhibits “remarkable” charge–discharge properties.."

Panasonic gets things done.

Remember that GM/Chevron spit in Panasonic's face, closing the Panasonic EV-95 NiMH battery assembly line for RAV4 EVs and taking $32 million in fines.

What Harvey said, the first forming cycles have big drops.. this sounds like a very low cost cell to manufacture..and it was done purely from theoretical calculations, amazing.

500 cycles in a 200 mile range EV is 100,000 miles.

Start with 200 mile range and get a reliable 160. That's enough for the occasional all day drive if rapid charging stations are available.

And, looking at the curve, the rate of decline seems to be decreasing so this would likely be a "one battery pack for the life of the vehicle" battery. (Calendar cliff possible.)

Since lithium titanate and LiFePo are both in the thousands of cycles before they decline to 80%, I don't really see what there is here to get enthusiastic about.

A) "Even at 30 C, which corresponds to a time of 2 min to fully discharge, the capacity was about 90% of that at the 1 C rate, implying that the battery is suitable for high-power applications." B) "Even after 500 charge−discharge cycles at the rate of 1 C (0.2 C every 20 times), 83% (first cycle, 231 mAh/g; 500th cycle, 193 mAh/g) of the capacity of the material was retained"- I'm no expert on battery chemistry or characteristics, and I find these conclusions confusing. Does (A) mean that when they discharged at the 30C rate, they obtained 90% of the work done by a battery fully discharged at the 1C rate? Or are they claiming the cycle life by a battery routinely discharged at the 30C rate is commensurate with that of their 1C test rig (as in (B)?

DaveM,

I was assuming the big news here was that these should be cheaper to produce than our favorite Toshiba batteries??? And hopefully higher energy density as well.

But as Bob Wallace points out, the degradation curve seems to be leveling off...I wonder what it would do at 1,000 cycles. 70%? 50%? It would be nice to know that.

The cycle life at restricted depths of discharge is probably much greater than the value quoted here.

The high power characteristic of this cell suggests its natural vehicle market is in conventional hybrids.  A 2 kWh battery charging and discharging at 30 C would handle 60 kW, and the energy to accelerate a 1500 kg vehicle to 100 kph is only 161 Wh, or 8% depth of discharge (plus losses).

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